First you do not have 4 Six volt batteries, You have two, two piece, 12 volt batteries (Please think of them as such)
now your question becomes "Is it possible to replace two 12 volt batteries with two 12 volt batteries".. Clearly the answer is yes I might add many other questions become as clear if you just think of those six volt batteries as 1/2 of a 12 volt each (The pair is a 12 volt size 4D in case you wonder).
Now: Why?
The Six volt are typically GC-2, 220 Amp Hour +/= 10 at 12 volts when paired,
A group 24 is 75 amnp hour give or take,m 27 just under 100, 29, just over
A group 31, is more. but to get the same amp hours you have now you';d need two 4D, and you will be spending way more dollars per amp hour for your batteries.